Volume control of transmission



'vDeco H. NYQUlsT VOLUME CONTROL OF TRANSMISSION Filed Sept. 15, 1924 2 Sheets-Sheet `l IN VEN TOR www A TTORNE Y Dec. 15, 1925.'l 1,565,491 J H. NYQUlsT VOLUME CONTROL OF TRANSMISSION Filed Sept. 13. 1924 2 Sheets-Sheet 2 ww. I I .l I M 5F14 54PM IN VE/Y TOR Zig/fum m ORNE Y Patented Dec. 15, 1.925.

UNITED STATES 1,565,491 PATENT oFFicE.

HARRY NYQ'UIST, OF JACKSON HEIGHTS, NEW YORK, ASSIGONOR T0 AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION 0F NEW YORK.

VOLUME CONTROL OF TRANSMISSION.

Application filed September 13, 1924. Serial No. 'v737,585.

To all whom t may concer/n.'

Be it known that-I, HARRY NYQUIST, residing at Jackson Heights, in the county of Queens and State of New York, have invented certain Improvements in Volume Control of Transmission, of which the following is a specification.

This invention relates to the transmission of signals, and more particularly to a meansfor and a method of changing the range of volume of the signals'during transmission.

- In telephone transmission, and in other kinds of signaling as well, it is often desirable to transmit sounds, such as speech and music, which have an extremely wide range of volume. The transmitting medium, however, may be ill adapted for thetransmission of wide ranges of volume by rea# son of the fact that its volume range is limited, on one hand by interference from disturbing noises, an'd, on the other hand by the necessity of avoiding distortion due to overloading repeaters. This is particularly true of such types of transmission systems as radio transmission circuits and public address system.

In accordance with the present invention, it is proposed to adjust the volume range of the signals to be transmitted to bring' it within the allowable range of the transmission medium. This may be accomplished by changing the gain at the transmitting end of the system, at the same time producing a compensating change in the gain at the other end. I n other words, the transmis? Sion level will be raised at the transmitting end and correspondingly lowered at the re ceiving end, or vice `"versa, whenever the volume of the signal becomes lower or greater than the average volume.

In order that the transmission level vmay be adjusted at the receiving endfor compensating the change in transmission level at the transmitting end, some method of control must be exercised from the transmitting end over the gain control device at the distant end. This may be accomplished in accordance with the present invention by transmitting over the medium and along with the signal a control frequency which is outside the range necessary for signal transmission. lVlienever the gain control device is adjusted at the transmitting end, a corresponding change is made in the amchange in the amplitude of the control fre-` quency, thus causing it to set the adjusting apparatus into operation to bring the overall equivalent of the medium back to normal.

The invention may now be more fully runderstood from the following description when read in connection with the accompanying drawing, Fig. 1 of which illustrates an embodiment of the invention as applied to a telephone transmission line; Fig. 1 of which illustrates how the apparatus of Fig. 1 may be applied to a radio system', bFig. 12- of which is a curve illustrating the operation of the detector in Fig. 1, and Fig. 3 of which illustrates a modification of the arrangement of Fig. l.

Referring to Fig. 1, L designates an ordinary telephone transmission line having amplifiers A and A near the terminals thereof. The line L, if of sufiicient length, may, of course, have amplifiers at points intermediate its length, as is well understood in the art. At the transmitting station, the

voice or other signaling currents are applied to the line L through a band filter-F and through the amplifier A. The amplifier A is providedv with a potentiometer 10 of the usual type for adjusting the gain. thereof. A rectiier-ampliier combination RA for controlling the voice operated relay 11 is connected to the voice input of the filter F. The voice operated relay 11, for'purposes of illustration, is shown as comprising a solel noid so constructed that its armature will be attracted in proportion to the amplitude ofY the voice currents, thereby raising; and4 v The armature is the solenoid adjusts the potentiometer to detheam litude of the voice current becomes lower t an normal, the solenoid adjusts the potentiometer to increase the gain. ,Consequently, as the volumev of the signal in- Greases, the gain-is decreased, and vice versa,

so that there' is a tendency to lreepV the volume transmitted over the line at a fairly constant level.

provided at the transmitting station, said channel comprising a source G of alternating current having a frequency preferably outside of the voice range and a filter F for separating the controlling channel from the voice channel. The connections from the source G through the lter F are applied to the terminals of the potentiometer 10 in parallel with the voice connections. Consequently, the adjustment of the potentiometer 10, as the volume of the voice current transmitted rises and falls, has the effect of decreasing or increasing thel magnitude of the controlling current as it appears on the output side of the amplifier .A. Accordingly, the amplitude of the control frequency arriving at the receiving station will be decreased or increased in proportion to any changevin' the adjustment of the amplifier 4A ofy the transmitting station.

At the receiving station a similar amplifier A1 is provided, said amplifier having an adjustable potentiometer 12 for controlling the gainl of the am lifier. The voice currents are selected by a lter F1 and transmitted to the voice channel'while the controlling frequency is selected by the filter F1 and transmitted to a detector D1. The detector D1 includes a relay It1 in its output circuit, and this relay is so adjusted that when the/normal amount vof controlling current is received from th line, itsrarmature does not rest on either contact. When the current received from the line is decreased, the armature closes one contact of the relay, and

when the current is increased, it closes the other contact. This o eration of the relay I' will be clear from the etector characteristic as shown by the curve of Fig. 2. The

'curve of Fig. 2 is obtained by plotting the alternate signaling currents (or potentials) applied to the detector as abscisszn, and the plate currents of the detector as ordinates.

-lin order to make the curve of Fig. 2 as steep as possible the grid of the detector is riormally biased suiiiciently negative so that the applied alternating current must attain considerable magnitude before any plate current flows.

By making the curve steep asv shown thedetector becomes very sensitive to slight changes in relativemagnitude of the applied current. The contacts of the .relay lit, control'the circuits of stepping magnets 13 and 14.- whiclrcontrol a ystep by step mechanism 15 to rotate a shaft .16 carrying an A'nauxiliary or' controlling channel transmitting endv becomes weaker than nor- The controlling current, therefore, arrives at the receiving end with increased volume, and upon being transmitted through the filter F to the detector D1 causes an increase in the steady current flowing through the Winding ofthe 'receiver R1. rlhe armature of the relay R, is accordingly moved to engage one of its contacts, lsay the upper contact, thereby completing a circuit through the winding of the stepping magnet 13 and through the interruptingrelayv 18. Impulses are now transmitted through the winding 13,thereby stepping the escapement wheel 15 to rotate the shaft 16`in such a direction that the potentiometer l2 is automatically adjustedto decrease the gain of the amplifier A1. This adjustment continues until the amplitude of the controlling current impressed upon the detector D1 is again restored to normal when the armature of the relay R1 will move back to its neutral point. The potentiometer 12 will now be so set that the amplitude of the received voice current will have the same value as it would have had if the transmission had been direct without any increase in gain at the transmitting station and corresponding increase in gain at the receiving station.

If the current in the voice channel at the transmitting end increases in volume, the solenoid 11 automatically operates to adjust the potentiometer 11 to reduce the gain of the amplifier A. The decrease in the gain of the amplifier A produces a correspondingv decrease in the amplitude of the controlling frequency -from thev source G, which accordingly arrives at the receiving stationv reduced in volume. The received controlling frequency atthe receiving station is transmitted through the filter F1 to the detector D1, and owing to its decrease iiiv volume produces a' decrease in the steady direct current` flowing through the winding of the relay R1. 'lhne armature of the relay is thereforev shifted to its lower contact,

`completing an interrupting circuit through the stepping magnet 14, 'which new steps the gain of the amplifier A1. As the gain of the amplifier increases, the amplitude of the controlling current increases until it reaches its normal value when the armature of the relay R1 will again assume its neutral position, and the adjustment of the shaft 16 will cease. The gain ofthe amplifier A1' will now be so set that the amplitude of the voice current transmitted through the lter `F1 will be increased in proportion to the decrease in gain produced by the adjustment of the potentiometer at the transmitting station. The over-all equivalent of the circuit is again the same as it was before the potentiometer at the transmitting end functioned.

It will be clear from the above operation that although the volume range of the voice current transmitted through the filter F may be quite wide, the volume range transmitted over the line L will be much narrower owing to the adjustment of the gain of the amplifier A. It will also be clear that the narrower range transmitted over `the line will be translated into a volume range for transmission through the filter F1 equal to the volume range transmitted through the filter F at the transmitting station. If the transmission equivalent of the line L should vary, due to changes in temperature or the like, the volume of: the control frequency from the source G as it arrives at the receiving-station will be correspondingly changed, thereby causing the relay R, to close one or the other of its contacts and automatically adjust the potentiometer 12 until the received controlling current impressed upon the detector D1 is restored to its normal value.

In other words, whenever the line equivalent Varies from normal the equipment at the receiving end functions to restore the over-all equivalent to its normal value.- vConsequently the system above disclosed'has the property of not only narrowing the'range .of volume transmitted from .the transmission medium but of maintamlng the transmission equivalent of the medium at a con'- stant value. v t

While the invention as above described 1s Ashow'n applied to a lineY transmission medium, the invention is equally applicable to radio transmission, as illustrated v1n Fig. 1, in which the radio transmitter RT and its associated antenna, together with vthe radio receiver vRR and its associated antenna, take' the place' of the transmission line of Fig. 1.

v; The operation of the apparatus of Fig. 1* will be apparent without further description.

lnf the case of line transmission, the volume range transmitted is limited, on the one hand, by the fact that the amplitude of the signaling current must bekept greater than 'the line" or other interfering noise, while the maznimum volume'mustnot exceed theimitfaboirewhich overloading ofthe -ples herein disclosed -may be embodied in tubes takes place. In the case of radio transmission, the upper volume limit will be determined by the overloading of the transmitting and receiving tubes, while-the lower limit will be determined by static and other interfering noises.

It yshould be'noted that the filter F ,l in addition to Lperforming its selecting function, acts as a delay network lto enable the automatic apparatus to adjust the potentiometer 10 in response to a change in volume, before thevoice Wave, in response to which the adjustment is made, has passed beyond the potentiometer. `It Will be noted, of course, that in order to accomplish this result the filter F may have a larger number of sections than the filter F in order to delay the wave passing through the filter F more than the wave passing through the filter F.

A similar result may be accomplished at thc receiving station by the modified arrangement illustrated in F ig; 3. Here, the amplifier A, andv potentiometer 12 vare arranged in the voice channel beyond the lter F1 and a corresponding amplifier Af, with its potentiometer 12', is arranged in the control channel beyond the filter F1. The potentiometers 12 and 12 are mechanically connected together to be voperated by the arm its normalvalue, the armature of the relay 'R1 will bein neutral position and the adjustment will cease. The potentiometer 12 will now be so adjusted that the change in volume introduced by rthe am lifier will just compensate .for the change o volume introduced by the amplifier A and potentiometer 10 at the transmitting station.

By arranging the receivingapparatus in the manner above described. the lter F1 may be so constructed as to delay'the trans- Y mission of the voice currents -to the potentiomv eter 12 and amplifier A1 until the 'escapement device 15 has been operated to adjust the potentiometers 12 and 12 in response to a change in volume. A wave corresponding to a change in' volume will therefore just A l arrive at the potentiometer l2 at the time the potentiometer has been, adjusted to compensate for the changein vlume.

It will be obvious that the general princimany other organizations widely dierent from those illustrated without departing from thespirit of the invention as defined in the following claims.

What is claimed is: e

l. In a signaling system, a transmission medium, an adjustable transmission element at the transmitting end of said medium, means to apply signaling currents to said transmission element, means to adjust said transmission element in accordance with the variations in volume of the'signaling current, means to applya control frequency to said transmission element, a compensating ransmission element at the receiving station, and means responsive to a change in amplitude of the received control current to adjust said compensating transmission element to compensate for the change in amplitude of the received control current.

2. In a signaling system, a transmission medium, an amplifier at the transmitting end of said `medium, means to impress signaling currents upon said medium through said amplifier, means responsive to the change in amplitude of said signaling currents to adjust the gain of said amplifier, means to impress a controlling current upon said medium through said amplifier so that said controllingcurrent will arrive at the receiving station with an amplitude determined'by the change in the gain of the amplifier at the transmitting station, a compensating amplifier associated with said medium at the receiving station, and means at said re'ceiving station responsive to any change in the amplitude of the received controlling current to produce a complementary adjustment of the gain of the amplifier of said receiving station. I

3. In a signaling system, a transmission medium, an amplifier associatedv with said medium vat the transmitting station, means to impress signaling currents on said medium through' said amplifier, means responsive to a change in the volume of the signaling currents to produce a change in the gain of said amplifier, means to impress a controlling current upon said medium through said amplifier, said controlling current \being transmitted over the medium and arriving at vthe receiving station with an amplitude determined by the change in the gain,l

of said amplifier land any change in the transmission equivalent of said medium, a? compensating amplifier associated with said medium at the receiving station, and means I responsive to any change in the amplitude of' the received control current to produce a compensating adjustment of' the gain of the amplifier at said receiving station whereby the transmission equivalent of the system will be maintained substantially constant.

4. In a vsignaling system, a transmission medium, an adjustable transmission element current, means to impress a controlling cur- A rent uponjsaid medium through said `element whereby said controlling current will arrive at a receiving station with an amplitude determined by the adjustment of said element and by Aany change in the transmission equivalent of said medium, a compensating adjustable transmission element associated with said medium at the receiving station, and means responsive to any change in the amplitude of said received control current to produce a compensating adjustment of the transmission element at the receiving station whereby the lover-all transmission equivalent of the system will. be maintained substantially constant. j

5. In a signaling system, a transmission medium, an .amplifier associated with said medium at Ya transmitting station, means to impress signaling currents upon said medium through said amplifier, means responsive to a change in the volumelof said signaling currents to adjust the gain of said amplifier, means to impress acontrolling current upon said medium through said amplifier, a compensating amplifier associated" with said transmitting medium at the receiving station, a signal channel and a control channel associated with `the output sidev of the amplifiers at the receiving station, means toselect the signaling currents into the signaling channel, )means to select the controlling currents into the controlling channel, and means responsive to a change in the amplitude of the selected controlling current to produce a compensating adjustment of the gain of the receiving amplifier. f

6. In a signaling system, a transmission medium, an adjustable transmission element associated with said medium at the transmitting station, means to impresssignaling v select signalingcurrents transmittedl through the transmission element at 'the receiving station into thegsignal channel, means I to select` control currents transmitted through said transmission element` at the receiving station into said control channel,

and means responsive yto changes in' the' amplitude of the Selected control current to produce a compensating adjustment of the transmission element at the receiving station.

7. In 'a signaling system, a transmission medium, and adjustable transmission ele` ment at the transmitting end of said medium, means to apply signaling currents to said transmission element, means to adjust said transmission element to keep the variations in volume of the signaling current transmitted to the mediu-,m within prescribed limits, means to apply a control frequency to said transmission element, said controllingfrequency being translnitted over the medium and arriving at the receiving. stationA with an amplitude determined by the adjustment of said transmission element, a compensating transmission element at the receiving station, and means to adjust said element in accordance with the amplitude of the received controlling frequency so that the transmission level will be raised when the received controlling frequency falls be low normal and will be lowered when the received controlling frequency is above normal.

8. In a signaling system, a transmission medium, an amplifier at the transmitting end of said medium, means to impress signaling currents upon said medium through said amplifier, means to adjust the gain of said amplifier to keep the volume of the signals transmitted to said medium within prescribed limits, means to impress a controlling current upon said medium through said amplifier so that said controlling current will arrive at the receiving station with an amplitude determined by the change in the gain of the amplifier at the transmitting station, a compensating amplifier associated with said medium at the receiving station, and means to adjust the gain of said last mentioned amplifier so that the gain will be increased when the amplitude of the received controlling current is below normal and the gain will be decreased when thel amplitude of the received controlling current is above normal.

9. In a signaling system, a transmission medium, an amplifier associated with said medium at the transmitting station, means to impress signaling currents upon said medium through said amplifier, means to adjust the gain of said amplifier to keep the volume of the transmitted signaling currents within prescribed limits, means to impress a controlling current upon said medium through said amplifier, said controlling current being transmitted over the medium and arriving at the receiving station with an amplitude determined by the change in the gain of said amplifier and any change in the transmission equivalent of said medium, a compensating amplifier associated with said medium at the receiving station, and means to adjust the gain of said last mentioned amplifier in accordance with the amplitude of the received controlling current so that the gain will be increased when the received controlling current falls below normal and will be decreased when the received controlling current is above normal, thereby maintaining the'transmission equivalent of the'system substantially constant.

l0. In a signaling system, a transmission medium, an adjustable transmission element associated therewith at the transmitting station, means to impress signaling currents upon said medium through said element, means to adjust said elements to keep the volume of the transmitted signaling current within prescribed limits, means to impress a controlling current upon said medium through said element whereby7 said controlling current will arrive at the receiving station with an amplitude determined by the adjustment of said element and by any change in the transmission equivalent of said medium, a compensating adjustable transmission element associated with said medium at the receiving station, and means to adjust said transmission element to raise the transmission level when the received controlling current is below normal and to lower the transmission level when the received controlling current is above normal, thereby maintaining the over-all transmission equivalent of the system substantially constant.

Y In testimony whereof, I have signed my name to this specification this 9th day of September, 1924.

HARRY NYQUIST. 

